Among the various types of belt retractors in use in vehicle passenger seat belt restraint systems are those which lock in response to a sharp or abrupt pull on the belt, such as that which occurs when the passenger is thrown forward abruptly in the event of a collision or upset of the vehicle, and those which are locked in response to the acceleration of a sensing pendulum. Some retractors are equipped with both of the aforementioned types of locking systems. For example, U.S. Pat. No. 3,991,953, issued Nov. 16, 1976, for "Automatic Locking Safety Belt Retractor" and assigned to Takata Kojyo Co., Ltd., describes and illustrates a retractor having both pendulum and reel inertia sensing devices which share certain elements and employ a second ratchet and pawl which are activated by either a pendulum or a reel inertia mechanism to engage a main pawl with a main ratchet wheel. The belt retractor of that patent can be designed to allow the belt to be pulled out freely, even at a relatively high rate of withdrawal, without locking while still locking in response to a relatively low acceleration of the pendulum (i.e., shifting of the pendulum upon acceleration or deceleration of the vehicle in an emergency situation) by means of the pendulum activated locking system.
In some cases, it is advantageous to mount a seat belt retractor on a vehicle door. This is particularly true in the case of passive restraint systems which are now coming into use, will be more widely used in the future, and will probably be required by government regulations. In addition to the convenience and effectiveness of the passive systems, the mounting of belt retractors on vehicle doors, particularly mounting between the inner and outer door panels, conserves space in the passenger compartment, another desirable aspect in the case of small automobiles.
One problem with a door mounted retractor is the possibility of the retractor being unintentionally locked when the door is swung open rapidly. Both pendulum actuated and inertially actuated retractors may be subject to locking in response to rapid door motion, thus abruptly stopping the door from opening further after locking and pulling the belt tightly across the passenger. In general, it is undesirable to make adjustments in a pendulum locking device or a reel inertia locking device, or both, to permit abrupt door movement because to do so would compromise the effectiveness of the locking retractor in an emergency situation.